Generally paper machine dryers (for which the present invention is particularly useful) are steam heated and are divided into a plurality of different dryer sections each composed of a number of dryer drums (cans) connected in parallel to a source of steam at a preselected pressure (temperature). In each drum the steam condenses thereby forming condensate and releasing heat for drying the paper. The released heat is transferred to the wet paper passing over the outside of the drum through the layer of condensate formed in the drum and the drum shell. The heat transfer rate through the condensate layer is about 8 to 10 times less than the rate through the shell of the dryer drum. Thus for efficient heat transfer (paper drying) the depth of the condensate layer inside the rotating drum (known as the rimming depth) should be maintained as small as is practical. In modern machines the rimming depth is about 1/16 inch.
The condensate is removed from each of dryer cans via a dryer syphon which consists of a shoe and a syphon pipe. Some steam also enters the dryer syphon with the condensate. This steam is known as blow through steam and assists the removal of the condensate. The steam and condensate from each dryer can discharge to a return header which in turn discharges to a separator tank. In this tank, the steam and condensate are separated. The condensate is removed from the tank by the condensate pump working on level control and is pumped to the steam plant. The blow through steam leaves the top of the tank and is piped to the suction side of a thermo compressor. The blow through steam, at a lower pressure than the supply steam is entrained by the motive steam entering the thermo compressor at a higher pressure than the supply steam and is discharged at the preselected supply steam pressure. The discharge steam from the thermo compressor consisting of the sum of the blow through steam and the motive steam, plus any make up steam, constitutes the supply steam required by the dryer section. The ratio of the mass flow of suction (blow through) steam to the mass flow of the motive steam is known as the entrainment ratio. For any given motive steam pressure and supply steam pressure, the entrainment ratio will decrease as the difference between the supply steam pressure and suction steam pressure increases. If the thermo compressor is unable to entrain all the blow through steam, (i.e. the pressure of the blow through steam is low as determined primarily by the pressure drop required to carry the condensate from the drum) the excess low pressure blow through steam (blow down steam) is discharged through the blow down valve and condensed and the steam required by the dryer section is met by increasing the amount of make up steam. It is thus evident that to minimize blow down, the entrainment ratio of the thermo compressor should be high and thus the difference in pressure between the supply steam and suction steam (pressure differential) should be maintained as small as possible.
The pressure loss caused by the flow of steam and condensate through the dryer system as a result of overcoming friction and centrifugal force (pressure drop for condensate removal) is a significant contributor to the differential steam pressure. Thus reduction of pressure loss through the dryer system can contribute to increased energy savings in paper machine drying operations by increasing the entrainment ratio of the thermo compressor and reducing the blow down potential.
To maintain the rimming depth of condensate at its optimum depth (generally about 1/16" for efficient heat transfer), the condensate must be removed from the dryer can through the syphon at the same rate as it is formed. If this is not achieved the rimming depth will increase and the dryer will flood and become inoperable. Dryer drums rotate at significant angular velocity and the resulting centrifugal force maintains the condensate in a rimming condition on the inside face of the drum. Removal of condensate from the drum requires overcoming the centrifugal force tending to prevent the flow of the steam condensate mixture toward the axis of rotation and of course frictional forces.
Many different types of condensate removal shoes have been used. In each case the condensate is forced into the shoe and through the condensate return pipe via the pressure difference between the steam pressure inside of the drum and in the separator tank.
Dryer shoes having internal plates or baffles to redirect the flow of steam and condensate entering the shoe in an axial or circumferential direction to a direction substantially perpendicular thereto are known. Examples of such devices are shown in U.S. Pat. Nos. 4,384,412 issued May 24, 1983 to Chance et al. or 4,516,334 issued May 14, 1985 to Wanke or 4,718,177 issued Jan. 1, 1988 to Haeszher et al.
U.S. Pat. No. 4,606,136 issued Aug. 19, 1986 to Pflug discloses a rotary syphon system wherein the transition between the inlet gap and outlet conduit is relatively smooth and the area of the transition passage is maintained substantially constant. To offset any tendency of the shoe to flood, additional steam is admitted to an internal chamber through ports holding the inner transition piece to the outer shoe. Steam enters these ports and appears as blow through steam at the separator tank of the dryer section regardless of whether any of the cans in the dryer section is flooded.
None of these devices nor any of the dryer syphon shoes of which applicant is aware address the shoe perimeter and clearance required to maintain efficient heat transfer in a dryer can nor the use of distributed flow in transporting the condensate through the syphon, i.e. the cross sectional area of the required passage through the syphon to convey the steam and condensate from the shoe perimeter to the exit pipe under distributed flow conditions.
None of these devices nor any syphon shoes of which applicant is aware coordinate the syphon inlet gap dimensions with the conditions within the dryer section to obtain distributed flow of condensate and steam entering the syphon from the drum so that the ratio of steam to condensate removed from the system through the gap is that required to obtain distributed flow using a minimum excess of steam (plus a factor of safety). Distributed flow lowers the pressure drop needed to convey the condensate from the perimeter of the drum (rim) to the axis of rotation.
There is a plethora of terms used in the literature to describe the various types of flow regimes that may be encountered in bi-phase flow. The definition of "segregated" versus "distributed" flow as used herein are as follows:
Segregated flows: flows where the liquid and gas phases are both essentially contiguous in the axial direction. Types of such flow are annular, crescent, wavy, stratified, etc. PA1 Distributed flows: flows where one phase is continuous and the other dispersed to one degree or another. This flow type includes dispersed flow (liquid distributed in vapour), bubble flow (vapour in liquid), froth flow, etc. PA1 P.sub.g =design steam pressure to the drum - psig PA1 R=Radius of the inside surface 74 of drum - ft. PA1 S=peripheral speed of the shell 22 - ft/min PA1 A=Area of perimeter of dryer can - sq. ft.
The concept of distributed flow of steam and water has been investigated and reported upon for example by A. E. Dukler in a book entitled "Gas-Liquid in Pipelines, 1. Research Results" dated May, 1969 produced for the American Gas Association, Inc., University of Houston and the American Petroleum Institute. Dukler has developed a model for predicting flow regime transition in horizontal and near horizontal gas liquid flows. See `A Model for Predicting Regime Transition in Horizontal and Near Horizontal Gas-Liquid` AlChE Journal (Vol., No. 1), January, 1976, pages 47-54 inclusive by Taitel and Dukler of the Department of Chemical Engineering, University of Houston.
Beggs et al. in `A Study of Two-Phase Flow in Inclined Pipes` published in the Journal of Petroleum Technology in May, 1973, pages 607-617 teaches how to determine pressure drop and liquid hold-up of two phase gas liquid flows in inclined pipes. These teachings were of particular interest to the petroleum, chemical and nuclear industries and were developed for predicting the flows of such chemicals.